The original one is so big........and the aftermarket one is so tiny. I'd think the big, original style was designed that way for a good reason........what's real the difference between the two in relation to how it affects shifting? o No:
The large older style OEM modulators work better with larger cams because the larger diaphragm surface area is more sensitive to low vacuum conditions. I would have to double check but I believe I can still get the large style new.
Jake, Was it something to do with altitude compensation? I seem to remember reading something along those lines in the chassis manual. Let us know if the large can are still available. I think I have one or two old ones. Can they be adjusted and then sealed at the adjustment screw with silicone?
They are still available. They are $20 each and are adjustable. I'm not sure if it was an altitude compensating deal. I don't have any literature on that. It might be mentioned in Ron Sessions TH400 book.
Where are they available? Everywhere I have looked it's the small replacement version. Seen a couple of NOS ones go for stupid money on ebay lately. I guess all you have to do is place the name "Corvette" in the listing to get more money. :dollar:
I would like to know where to get the larger ones also. I put my original back on my SP 400 and the shifts where much better/firmer. Clyde
I own a small performance/muscle car shop and specialize in GM automatic transmissions. I get my parts from several suppliers. I'm not sure who is making these. They are large canister style replacement of the "OEM Black" modulator. I think they are gold coated like the smaller ones. I'm not sure where you would get them other than a transmission shop or wholesale supplier. They don't pull up under the generic TH400 parts list for me, I had to do some searching to find them. They are $20 jobber price and almost $30 list. They may be a Rostra brand. I'll order one in and check it out.
I actually have 2 of the larger ones.......I've never tried the small one though, I was just wondering what was up with that. uzzled:
The large modulators had an aneroid (spelling?) bellows in them that compensated for changes in altitude. I believe that under less barometric pressure the bellows would expand and act against the spring pressure in the small end (with the screw) to prevent harsh shifts at higher altitudes. The early (up to 1972) ones were black and later ones on EGR cars were dark blue. The bellows section was smaller on the blue ones as the engine vacuum was less on EGR engines and the modulators were calibrated differently. Basically, all ST300, TH350 and TH400 used similar modulators however there were differences in calibration. Most likely the replacements today are a "one fits all" part number. Here is a link to the black one available at Advance Auto Parts for about $22. http://www.partsamerica.com/ProductDetail.aspx?MfrCode=SMP&MfrPartNumber=TM17&PartType=1159&PTSet=A Scott
Weird. I have done a search on the Parts America site for them before, (the last time being when Joe posted this thread) And that one never came up. Good to know, I'm gonna pick one up. :TU:
Problem is originals had vacuum port on side and factory metal line fit inside with rubber hose over it. The one in picture is in front like the small one and you need a longer piece of hose to snake around to the opening and hose is prone to falling off and also if you have headers it comes real close to it.
The one pictured is exactly like an original. The brass ones have the vacuum nipple on the end. That is why they come with a molded vacuum hose. The metal line comes straight down and needs to make a turn to fit into the modulator. This one has the vacuum nipple perpendicular to that. The line comes straight down into the nipple.
Thanks Larry. I did not enlarge the picture and though the blue end was the nipple. I will have to orfer one of those as I have been looking for one.
